The present invention relates to an optical interference signal extractor for use in measuring the spectral distribution of light.
FIG. 1 shows a conventional optical interference signal extractor. Reference numeral 10 indicates a light source for emitting light 11 to be measured and 20 an optical interferometer capable of sweeping an optical path difference, that is, capable of continuously changing an optical path difference. In this example the optical interferometer 20 is shown to be a Michelson interferometer but may also be a Fabry-Perot interferometer.
The Michelson interferometer 20 comprises a half mirror or semitransparent mirror 21 for splitting and combining light, a fixed mirror 22 for forming a fixed optical path 24, and a movable mirror 23 for forming a variable optical path 25.
The light 11 emitted from the light source 10 is split by the half mirror 21 to the fixed optical path 24 and the variable optical path 25, and light from the former and light from the latter are coupled together or combined by the half mirror 21 into a beam of light, which is converted by a photodetector 30 into an electric signal SA.
The light having passed through the fixed optical path 24 and the light having passed through the variable optical path 25 have a phase difference owing to the optical path difference, and hence interfere with each other when they are coupled together by the half mirror 21. The interference of light varies as the movable mirror 23 moves. In other words, the intensity of the interference light from the optical interferometer 20 repeatedly varies corresponding to interference fringes as the movable mirror 23 moves. The intensity variation of the interference light corresponding to the distance of movement .DELTA.l of the movable mirror 23 from the position where the optical path difference is zero is measured by conversion into the level of an electric signal by the photodetector 30, whereby such an optical interference signal SA as shown in FIG. 2 can be obtained. The movement of the movable mirror 23 for the distance .DELTA.l causes an optical path difference of 2.DELTA.l, but in the following description, the distance of movement .DELTA.l may sometimes be referred to as the optical path difference for the sake of simplicity. By a frequency analysis of the interference signal SA through Fourier transformation, the spectral distribution of the light to be measured 11 can be obtained. Where the moving speed of the movable mirror 23 is made constant, time t corresponding to the distance of its movement may be represented on the abscissa in FIG. 2.
FIG. 2 shows the waveform of the interference signal SA when the optical power of the light to be measured 11 is stable. Where the optical power (indicated by SB in FIG. 2) of the light 11 is stable, an interference signal of a good SN ratio can be obtained. On the other hand, where the optical power SB of the light 11 fluctuates as depicted in FIG. 3, the SN ratio of the interference signal SA becomes poor, and the results of its frequency analysis contain, as spectral, also high-order modulation components resulting from the fluctuation in the optical power of the light 11, making it impossible to accurately measure the wavelength distribution in the light 11.